Method of manufacturing oil cooler for automatic transmission

ABSTRACT

A method of manufacturing an oil cooler for an automatic transmission includes: forming an inner pipe and an outer pipe, welding the plate and cutting the pipe; forming a boss using a cold forged rod; forming a brazing washer; forming a flow hole in both ends of the outer pipe and pass oil through the flow hole; compressing and fixing the brazing washer and the boss; forming a heat radiation fin; assembling the outer pipe in which the boss is fixed, the heat radiation fin plated with copper, and the inner pipe; sequentially expanding the inner pipe at its both ends in the radial direction; closely contacting the heat radiation fin with the inner pipe and the outer pipe; mounting brazing rings in the both expanded ends of the inner pipe and mounting a workpiece of the oil cooler in a brazing jig.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing an oil cooler for an automatic transmission, and more particularly, to a method of manufacturing an oil cooler for an automatic transmission that can be made in a mass production system because a process of joining parts of the oil cooler by furnace brazing is performed and therefore an acid processing process and a heat treatment process become unnecessary.

2. Description of the Related Art

A conventional method of manufacturing an oil cooler for an automatic transmission is performed according to following processes described below:

1) a process of forming an outer pipe by cutting a copper or brass pipe in a predetermined size;

2) a process of forming an inner pipe by cutting a copper or brass pipe having a diameter smaller than that of the outer pipe in a predetermined size;

3) a process of forming a boss in a predetermined processing size using a brass rod;

4) a process of forming a flow hole in both ends of the outer pipe in order to easily mount the boss and pass oil through the flow hole;

5) a process of fixing the boss to the flow hole of the outer pipe by oxygen welding after mounting the boss in the flow hole of the outer pipe;

6) a process of cleaning an oxidized portion by treating it with an acid after performing the oxygen welding;

7) a process of forming a heat radiation fin by compressing a brass plate with a roller;

8) a process of assembling the heat radiation fin, the inner pipe, and the outer pipe in which the boss is welded;

9) a process of expanding both ends of the inner pipe in the radial direction and closely contacting the inner pipe with the outer pipe at the both ends of the inner pipe;

10) a process of expanding the intermediate portion of the inner pipe except for both ends of the inner pipe in the radial direction to closely contact the heat radiation fin with the inner pipe and the outer pipe;

11) a process of annealing a workpiece of an oil cooler in order to weld both ends of the workpiece by argon welding;

12) a process of welding the both expanded ends of the workpiece by argon welding;

13) a process of inspecting leakage in welded portions after mounting the welded workpiece on a leakage inspection device; and

14) a process of printing a production date on an oil cooler and packing the oil cooler in a standard box.

FIG. 1 is a perspective view illustrating a process of forming a heat radiation fin in a conventional process of manufacturing an oil cooler for an automatic transmission.

Referring to FIG. 1, a set of rolls 2 presses a brass plate 4 to form a heat radiation fin 6.

As described above, in a conventional process of manufacturing an oil cooler for an automatic transmission, an oxygen welded portion is oxidized because a boss is welded to an outer pipe using oxygen, and an acid treatment of the oxidized portion should be performed in the oxidized portion.

Further, in a state where a heat radiation fin is mounted between an inner pipe and the outer pipe, the heat radiation fin closely contacts with the inner pipe and the outer pipe only by expanding the inner pipe in the radial direction, and therefore heat radiating performance is deteriorated.

That is, in a state where the inner pipe, the outer pipe and the heat radiation fin are not welded, the oil cooler is manufactured only by expanding the inner pipe in the radial direction, and therefore heat radiating performance is deteriorated.

Further, when the inner pipe becomes closer to the outer pipe by expanding an intermediate portion of the inner pipe in the radial direction, the inner pipe is expanded once by a ball punching, and thus an amount of expansion of the pipe is limited.

Further, a closed space exists between the inner pipe and the outer pipe at their both ends, and when a gap of the closed space is large, the inner pipe may be broken during a pipe expansion process and thus a mounting height of the heat radiation fin is limited.

Because argon welding is performed in the both expanded ends of the oil cooler, heat treatment of the oil cooler in an oven is required for a predetermined time period.

Further, because only a highly skilled worker can perform argon welding, a high production cost is resulted, and when argon welding is not appropriately performed, leakage may happen in an argon welded portion, and therefore a quality of the oil cooler is deteriorated.

Further, in a method of forming the heat radiation fin (refer to FIG. 1), because the heat radiation fin is formed using a set of rollers which are expensive and easily worn out, a high cost is resulted due to use of the roller.

Further, the heat radiation fin should be formed in various shapes according to required performance of the oil cooler, however it is difficult to form the heat radiation fin in various shapes when the heat radiation fin is formed in a rolling method.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, and provides a method of manufacturing an oil cooler for an automatic transmission that can be made in amass production system because a process of joining parts of the oil cooler is performed by furnace brazing and therefore an acid treatment process and a heat treatment process become unnecessary.

The present invention further provides a method of manufacturing a heat radiation fin for an oil cooler that can be manufactured with a low cost by forming the heat radiation fin with a press mold instead of forming the heat radiation fin by a rolling method.

The present invention further provides an oil cooler for an automatic transmission having an excellent heat radiating performance and a method of manufacturing the same by compressing a boss together with a brazing washer to an outer pipe to enable furnace brazing, and by melting a copper plated on a heat radiation fin to join the inner pipe and the outer pipe when the furnace brazing is performed.

The present invention further provides a method of manufacturing an oil cooler for an automatic transmission that can maximize expansion of a pipe by sequentially expanding the pipe in the radial direction by a multistage method using a turret type pipe expansion tool set instead of an existing taper type pipe expansion method when performing a pipe expansion process to close both ends of an inner pipe and an outer pipe after mounting a heat radiation fin between the inner pipe and the outer pipe.

In accordance with an aspect of the present invention, a method of manufacturing an oil cooler for an automatic transmission, includes: forming an inner pipe and an outer pipe by rolling a stainless steel plate in a circular shape, welding the stainless steel plate in a pipe of a standard specification and cutting the pipe in a predetermined length (S10); forming a boss using a cold forged stainless steel rod (S20); forming a brazing washer of a predetermined size by punching a copper plate (S30); forming a flow hole in both ends of the outer pipe to mount the boss and pass oil through the flow hole (S40); compressing and fixing the brazing washer and the boss to the both ends of the outer pipe (S50); forming a heat radiation fin by compressing a stainless steel or iron plate into a heat radiation fin forming mold (S60); assembling the outer pipe in which the boss is fixed, the heat radiation fin plated with copper, and the inner pipe (S70); sequentially expanding the inner pipe at its both ends in the radial direction using a plurality of turret type pipe expansion tool sets formed with a first division type forming jig and a second turret type forming jig (S80); closely contacting the heat radiation fin with the inner pipe and the outer pipe by expanding an intermediate portion of the inner pipe (S90); mounting brazing rings in the both expanded ends of the inner pipe and mounting a workpiece of the oil cooler in a brazing jig (S100); brazing the portions (bosses, closed portions at both ends, and a heat radiation fin) of the inner pipe and the outer pipe under optimum conditions (temperature, speed, gas flow rate, and cooling water flow rate) set for the portions (S110); inspecting leakage of brazed portions after mounting the oil cooler on a leakage inspection device (S120); and printing a production date on the oil cooler and packing the oil cooler in a standard box (S130).

Preferably, sequentially expanding the inner pipe at its both ends in the radial direction (S80) includes: providing the first division type forming jig to which a plurality of division surfaces are coupled and the second turret type forming jig inserted into and coupled to the first division type forming jig to expand the end of the inner pipe by pushing the first division type forming jig in a radial direction; arranging a plurality of first division type forming jigs by a multistage method in order to gradually increase a height of a portion contacting with the inner pipe; coupling the second turret type forming jig to the first division type forming jig by advancing the second turret type forming jig in a state of contacting the first division type forming jig having a smallest height among the first division type forming jigs with the inner pipe; forcedly dividing the first division type forming jig by continuously advancing the second turret type forming jig and thus expanding the end of the inner pipe in a radial direction; restoring the first division type forming jig to an original coupling state with a spring force by retreating the second turret type forming jig; and sequentially expanding the end of the inner pipe by forwardly and backwardly traveling the second turret type forming jig in a state of contacting another first division type forming jigs having a height larger than that of the initial first division type forming jig with the end of the inner pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will be more apparent from the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view illustrating a heat radiation fin forming process in a conventional process of manufacturing an oil cooler for an automatic transmission;

FIG. 2 is a perspective view illustrating a process of manufacturing an inner pipe and an outer pipe (S10) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 3 is a perspective view illustrating an outer pipe forming process (S40) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 4 is a perspective view illustrating a compression process of a brazing washer and a boss (S50) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 5 is a perspective view illustrating a process of forming and cutting a heat radiation pin formed with a stainless steel plate (S60) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 6 is a perspective view illustrating a process of assembling an oil cooler in order of an outer pipe, heat radiation fin, and inner pipe (S80) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 7 is a cross-sectional view illustrating a turret type pipe expansion tool set for expanding an inner pipe in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 8 is a cross-sectional view illustrating a coupled turret type pipe expansion tool set for expanding an inner pipe in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 9A-9E are views illustrating processes of sequentially expanding an inner pipe in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 10 is a perspective view illustrating an inner pipe expansion process (S100) of closely contacting a heat radiation fin with the inner pipe and the outer pipe in the process of FIG. 6;

FIG. 11 is a partially enlarged side cross-sectional view illustrating a portion A in the inner pipe expansion process of FIG. 10;

FIG. 12 is a perspective view illustrating a process of assembling a brazing ring and mounting a brazing jig (S110) and a brazing process (S120) after expanding the inner pipe in the inner pipe expansion process of FIG. 10;

FIG. 13 is a perspective view illustrating a leakage inspection process (S130) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention;

FIG. 14 is a perspective view illustrating a packing process (S140) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention; and

FIG. 15 is a block diagram illustrating a method of manufacturing an oil cooler according to another exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. The same reference numbers are used throughout the drawings to refer to the same or like parts. The views in the drawings are schematic views only, and are not intended to be to scale or correctly proportioned. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention.

While the present invention may be embodied in many different forms, specific embodiments of the present invention are shown in drawings and are described herein in detail, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.

FIG. 2 is a perspective view illustrating a process of manufacturing an inner pipe and an outer pipe (S10) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention, FIG. 3 is a perspective view illustrating an outer pipe forming process (S40) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention, and FIG. 4 is a perspective view illustrating a compression process of a brazing washer and a boss (S50) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention.

FIG. 5 is a perspective view illustrating a process of forming and cutting a heat radiation pin formed with a stainless steel plate (S60) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention, and FIG. 6 is a perspective view illustrating a process of assembling an oil cooler in order of an outer pipe, heat radiation fin, and inner pipe (S80) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention.

FIG. 7 is a cross-sectional view illustrating a turret type pipe expansion tool set for expanding an inner pipe in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention, FIG. 8 is a cross-sectional view illustrating a coupled turret type pipe expansion tool set for expanding an inner pipe in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention, and FIG. 9A-9E are views illustrating processes of sequentially expanding an inner pipe in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention.

FIG. 10 is a perspective view illustrating an inner pipe expansion process (S100) of closely contacting a heat radiation fin with the inner pipe and the outer pipe in the process of FIG. 6, and FIG. 11 is a partially enlarged side cross-sectional view illustrating a portion A in the inner pipe expansion process of FIG. 10.

FIG. 12 is a perspective view illustrating a process of assembling a brazing ring and mounting a brazing jig (S110) and a brazing process (S120) after expanding the inner pipe in the inner pipe expansion process of FIG. 10, FIG. 13 is a perspective view illustrating a leakage inspection process (S130) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention, and FIG. 14 is a perspective view illustrating a packing process (S140) in a process of manufacturing an oil cooler for an automatic transmission according to an exemplary embodiment of the present invention.

Referring to the drawings, in an oil cooler 10 for an automatic transmission, an inner pipe 14 is inserted into an outer pipe 12, and a space portion 16 is formed between the outer pipe 12 and the inner pipe 14. Both ends of the outer pipe 12 and the inner pipe 14 are closed.

In this case, a heat radiation fin 18 is inserted into the space portion 16, and both sides of the outer pipe 12 are formed to communicate oil, and a sheet connection portion 20 though which oil passes is assembled to both sides of an upper surface of the outer pipe 12 so that a tube and a boss 13 communicates with the space portion 16.

Both ends of the inner pipe 14 are expanded, the inner pipe except for the both ends is expanded by a ball expansion method, and an outer surface of the inner pipe 14 closely contacts with the heat radiation fin 18.

A method of manufacturing an oil cooler 10 for an automatic transmission having the above configuration is described in the below.

A pipe of a standard specification is manufactured by welding a stainless steel plate in a circular shape and the inner pipe 14 and the outer pipe 12 are manufactured by cutting the pipe in a predetermined length (S10).

Further, after cold forge forming with a stainless steel rod, the boss 13 is formed in a lathe according to a processing standard (S20).

A brazing washer 22 of a predetermined size is manufactured with a copper plate (S30).

A flow hole 24 through which oil can flow is formed in both ends of the outer pipe 12 and forms a horizontal plane in order to easily mount the boss 13 (S40).

Further, the brazing washers 22 and the bosses 13 are compressed and fixed to both ends of the outer pipe 12 (S50).

A mold 19 for forming the heat radiation fin 18 is manufactured in advance, and the heat radiation fin 18 is formed by applying a pressure from an upper side and a lower side of transferred stainless steel or iron plate to the mold 19, and the heat radiation fin 18 is cut in a predetermined length (S60).

In this case, after the heat radiation fin 18 is plated with copper in a plate state, a heat radiation pin forming process is performed and the plated copper is melted for brazing in a brazing process.

Assembly of parts is complete by coupling the outer pipe 12 in which the boss 13 is compressed, the inner pipe 14, and the heat radiation fin 18 in which copper is plated (S70).

The both ends of the inner pipe 14 closely contact with the outer pipe 12 by expanding both ends of the inner pipe 14 using turret type pipe expansion tool sets 25 a and 25 b (S80).

A method of expanding both ends of the inner pipe 14 is described in detail as follows.

(1) The turret type pipe expansion tool set 25 a and 25 b is provided to expand the inner pipe 14, and the turret type pipe expansion tool set 25 a and 25 b includes a first division type forming jig and a second turret type forming jig.

The first division type forming jig is coupled by a restoring spring in a state divided into six or eight equal parts. The first division type forming jig has a tapered space portion at the inside of its coupled structure, and an end of the outside of the first division type forming jig contacting with the inner pipe 14 is tapered by a predetermined angle.

The second turret type forming jig is inserted into and is coupled to the tapered space portion of the first division type forming jig, and the outside of the first division type forming jig has a tapered form for easy insertion.

The second turret type forming jig is temporarily coupled to the first division type forming jig to push the first division type forming jig divided into a predetermined number in a radial direction, thereby expanding an end of the inner pipe 14.

(2) A plurality of first division type forming jigs are provided and arranged by a multistage method so that heights of the first division type forming jigs of a portion contacting with the inner pipe 14 gradually increase.

(

3) In a state where the thinnest portion of portions contacting with the inner pipe 14 among the first division type forming jig contacts with the inner pipe 14, the second turret type forming jig is inserted into and is coupled to a taper groove of the first division type forming jig by advancing the second turret type forming jig.

(4) When continuously advancing the second turret type forming jig in this state, the first division type forming jig is divided into a plurality of pieces along a separation surface and an end of the inner pipe 14 is expanded while being pushed in a radial direction.

(5) Thereafter, a separation surface of the first division type forming jig is again contracted through a force of a spring 25 c by retreating the second turret type forming jig and the second turret type forming jig is returned to a temporarily coupled state.

(6) Thereafter, in a state where another first division type forming jig having a height larger than that of the initial first division type forming jig contacts with an end portion of the inner pipe 14, the end of the inner pipe 14 is further expanded by advancing the second turret type forming jig.

(7) The end of the inner pipe 14 is sequentially slowly expanded by this method, and in the present exemplary embodiment, total 5 first division type forming jigs are provided and the end of the inner pipe 14 is sequentially expanded.

That is, in the present exemplary embodiment, after mounting the heat radiation fin 18 between the inner pipe 14 and the outer pipe 12, a pipe expansion process of closing both ends of the inner pipe 14 and the outer pipe 12 is performed with a turret pipe expansion method instead of an existing ball type pipe expansion method, and specifically, a pipe expansion tool having a dual structure of a first division type forming jig divided into six equal parts and a second turret type forming jig is used.

Accordingly, after the first division type forming jig divided into six equal parts is firstly injected into the inner pipe 14, the second turret type forming jig is injected and thus an outer diameter of the first division type forming jig increases in a turret method, whereby the pipe is easily expanded.

By increasing expansion of a pipe on a stepwise basis by installing a turret type pipe expansion tool by a multistage method, a pipe expansion stress of the inner pipe 14 is gradually reduced and thus expansion of the pipe is increased and a use range of the heat radiation fin 18 can be widened.

FIGS. 9A to 9E illustrate a turret type pipe expansion tool set installed in a multistage, and ends of the first division type forming jig are disposed to gradually increase and in an initial state, a first division type forming jig having a small height t1 is installed and the inner pipe 14 is expanded in the radial direction.

Another first division type forming jig having a height t2 larger than that of the initial first division type forming jig is installed and the inner pipe 14 is further expanded in the radial direction and in this way, by sequentially installing first division type forming jigs having gradually increasing heights t3, t4, and t5, the inner pipe 14 is slowly expanded in the radial direction.

The heat radiation fin 18 mounted in the space portion 16 closely contacts with the inner pipe 14 and the outer pipe 12 by expanding the inner pipe 14 of an assembled oil cooler 10 to a predetermined size by a ball expansion jig 27 (S90).

Brazing rings 26 are mounted in both ends of the inner pipe 14 and the outer pipe 12.

A workpiece of an oil cooler 10 is mounted in the brazing jig 28 (S100).

In order to braze portions (bosses, closed portions at both ends, and a heat radiation fin) of the workpiece of the oil cooler 10 fixed to the brazing jig 28 under an optimum condition, conditions (a temperature, speed, gas flow rate, and cooling water flow rate) are set and brazing is performed (S110).

In this case, the heat radiation fin 18 is mounted between the outer pipe 12 and the inner pipe 14 and when brazing is performed, a copper component of the copper plated heat radiation fin 18 is melted for brazing with the inner pipe 14 and the outer pipe 12.

As described above, the oil cooler 10 in which brazing is complete is mounted in an leakage inspection device 30 and leakage of the brazed portions is inspected (S120).

Production and shipping preparation of the oil cooler 10 are finally complete by printing a production date in the oil cooler 10 in which the leakage inspection is complete and packing the oil cooler 10 in a standard box 32 (S130).

As described above, when the oil cooler 10 is manufactured, each part is formed with stainless steel having a small thickness, thereby remarkably improving thermal conductivity, corrosion resistance, and productivity.

Further, because all joining processes are performed by furnace brazing, the number of manufacturing processes are reduced, productivity is improved and thus a production cost is reduced.

As described above, according to the present invention, the following advantages are obtained.

(1) Because a joining process of parts of an oil cooler is performed by furnace brazing, an acid treatment process and a heat treatment process are unnecessary and thus the oil cooler according to the present invention can be made in amass production system.

(2) Because a heat radiation fin is formed using a press mold instead of forming by a rolling method, the heat radiation fin according to the present invention can be manufactured with a low cost.

(3) Furnace brazing can be performed by compressing a boss together with a brazing washer to an outer pipe, the heat radiation fin is plated with copper, and when furnace brazing is performed, the plated copper is melted for brazing with an inner piper and the outer pipe, thereby having excellent heat radiating performance.

(4) After the heat radiation fin is mounted between the inner piper and the outer pipe, a pipe expansion process of closing both ends of the pipes is performed, and the pipe is sequentially expanded by a multistage method using a turret type pipe expansion tool set, thereby maximizing an expansion of the pipe.

Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and modifications of the basic inventive concepts herein described, which may appear to those skilled in the art, will still fall within the spirit and scope of the exemplary embodiments of the present invention as defined in the appended claims. 

1. A method of manufacturing an oil cooler for an automatic transmission, comprising: forming an inner pipe and an outer pipe by rolling a stainless steel plate in a circular shape, welding the stainless steel plate in a pipe of a standard specification and cutting the pipe in a predetermined length (S10); forming a boss using a cold forged stainless steel rod (S20); forming a brazing washer of a predetermined size by punching a copper plate (S30); forming a flow hole in both ends of the outer pipe to mount the boss and pass oil through the flow hole (S40); compressing and fixing the brazing washer and the boss to the both ends of the outer pipe (S50); forming a heat radiation fin by compressing a stainless steel or iron plate into a heat radiation fin forming mold (S60); assembling the outer pipe in which the boss is fixed, the heat radiation fin plated with copper, and the inner pipe (S70); sequentially expanding the inner pipe at its both ends in the radial direction using a plurality of turret type pipe expansion tool sets formed with a first division type forming jig and a second turret type forming jig (S80); closely contacting the heat radiation fin with the inner pipe and the outer pipe by expanding an intermediate portion of the inner pipe (S90); mounting brazing rings in the both expanded ends of the inner pipe and mounting a workpiece of the oil cooler in a brazing jig (S100); brazing the portions (bosses, closedportions at both ends, and a heat radiation fin) of the inner pipe and the outer pipe under optimum conditions (temperature, speed, gas flow rate, and cooling water flow rate) set for the portions (S110); inspecting leakage of brazed portions after mounting the oil cooler on a leakage inspection device (S120); and printing a production date on the oil cooler and packing the oil cooler in a standard box (S130).
 2. The method of claim 1, wherein sequentially expanding the inner pipe at its both ends in the radial direction (S80) comprising: providing the first division type forming jig to which a plurality of division surfaces are coupled and the second turret type forming jig inserted into and coupled to the first division type forming jig to expand the end of the inner pipe by pushing the first division type forming jig in a radial direction; arranging a plurality of first division type forming jigs by a multistage method in order to gradually increase a height of a portion contacting with the inner pipe; coupling the second turret type forming jig to the first division type forming jig by advancing the second turret type forming jig in a state of contacting the first division type forming jig having a smallest height among the first division type forming jigs with the inner pipe; forcedly dividing the first division type forming jig by continuously advancing the second turret type forming jig and thus expanding the end of the inner pipe in a radial direction; restoring the first division type forming jig to an original coupling state with a spring force by retreating the second turret type forming jig; and sequentially expanding the end of the inner pipe by forwardly and backwardly traveling the second turret type forming jig in a state of contacting another first division type forming jigs having a height larger than that of the initial first division type forming jig with the end of the inner pipe. 